The invention relates to an infrared radiator with a heater disposed in an envelope tube of clear or opaque fused vitreous silica and a metallic reflective coating applied to the back of the envelope tube, and a method for its manufacture.
Infrared radiators whose heater is surrounded by a envelope tube of clear or opaque fused vitreous silica are disclosed, for example, in German Patents 1,540,818 and 38 41 448 (to which U.S. Pat. No. 5,003,284 corresponds). To reduce lateral and back radiation losses the envelope tube can be provided on its back with a reflective coating of metal, such as aluminum or gold. Infrared radiators of this kind are described also in the prospectuses of Heraeus Quarzschmelze GmbH "Kurzwellige Infrarotstrahler aus Hanauer Quarzglas" (PIR-B 20) and "Mittelwellige Zwillingsrohr-Infrarotstrahler" (PIR-B 10).
It has been found that the metallic reflective coatings are not sufficiently stable in infrared radiators operating under very severe loads, and are gradually destroyed.
One possibility for preventing the destruction of the reflective coating of an infrared radiator is disclosed in German Patent 26 37 338. The infrared radiator has, in addition to the envelope tube of transparent or opaque fused vitreous silica surrounding the heater, a cooling tube through which a coolant flows. The reflective coating is situated on the cooling tube and is thus protected against destruction by evaporation.